This invention relates generally to semiconductor packaging, and specifically to an improved semiconductor package having a metal foil die mounting plate. This invention also relates to a method for fabricating the package.
Plastic semiconductor packages are fabricated using lead frames. A conventional lead frame comprises a stamped, or etched metal strip, adapted to support multiple dice for packaging. The lead frame supports the dice during a molding process, and provides the internal signal traces and terminal leads in the completed packages. A conventional plastic package includes a semiconductor die attached to a portion of the lead frame, and encapsulated in a plastic body.
One consideration in designing a plastic package is heat transfer from the die. Operation of the integrated circuits within the die generate heat which must be dissipated. In a conventional plastic semiconductor package, the bulk of heat transfer from the encapsulated die is through the terminal leads of the package. However, as circuit densities increase, semiconductor dice generate additional heat, and the leads of the package are not always able to efficiently dissipate the heat.
This has led to the development of semiconductor packages that include a heat sink. Typically, the heat sink comprises a metal plate embedded in the plastic body of the package. Representative heat sink packages are described in U.S. Pat. No. 5,666,003 entitled xe2x80x9cPackaged Semiconductor Device Incorporating Heat Sink Platexe2x80x9d, and in U.S. Pat. No. 5,629,561 entitled xe2x80x9cSemiconductor Package With Integral Heat Dissipatorxe2x80x9d.
One aspect of packages that incorporate a heat sink is that the heat sink is typically an additional component. Accordingly, the heat sink must be incorporated into the package design, and tends to complicate the package fabrication process. Also, the heat sink typically only performs a heat transfer function.
It would be advantageous for a heat sink to perform other functions in the package, such as electrical and structural functions. In addition, it would be advantageous for a fabrication process to incorporate heat sinks into packages without requiring additional complicated steps. The present invention is directed to a package that includes a heat sink element that performs electrical and structural functions. In addition the present invention is directed to a simple fabrication process for forming a heat sink semiconductor package.
In accordance with the present invention, an improved semiconductor package, and a method for fabricating the package are provided. The package can be fabricated using a tape under frame (TUF) lead frame, or a leads under chip (LUC) lead frame. The package includes a semiconductor die wire bonded to lead fingers of the lead frame, and encapsulated in a plastic body. The package also includes a die mounting plate, which attaches to the lead fingers, and mounts the die to the lead frame.
The die mounting plate comprises a thermally conductive metal foil, about 1 to 4 mils thick, with an adhesive layer on one side for attaching the foil to the lead frame. The die mounting plate provides a direct thermal path from the die, to the terminal leads of the package. In addition, the die mounting plate can be configured as a ground plane, or as a voltage plane, for the package. Still further, the die mounting plate can be configured to adjust an impedance of internal signal traces of the package.
For fabricating the package, the die mounting plate is attached to a die mounting site on the lead frame using the adhesive layer. Next, the die is back bonded to the die mounting plate using a thermally conductive adhesive, such as silicone or epoxy. Next, the die is wire bonded to the lead fingers of the lead frame, and a plastic body formed around the die using a transfer molding process. Finally, the plastic body is separated from a remainder of the lead frame, and the terminal portions of the lead fingers shaped as required to form package leads.